1. Field of Invention
The present invention relates to thermoplastic olefin compositions and injection molded articles made thereof. More particularly, the present invention relates to thermoplastic olefin compositions that exhibit substantially isotropic shrinkage properties when used to form articles by injection molding.
2. Description of Related Art
Thermoplastic olefin (“TPO”) compositions are multiphase polymer blends that generally comprise a continuous polyolefin matrix phase having discrete domains of a discontinuous alpha-olefin copolymer elastomer rubber phase dispersed therein. TPO compositions undergo plastic flow when heated above the softening point of the continuous polyolefin matrix phase, which makes the TPO compositions particularly suitable for use in fabricating articles via injection molding processes. The continuous polyolefin matrix phase impart tensile strength and chemical resistance to injection molded articles formed of the blend, and the discrete domains of alpha-olefin copolymer elastomer rubber dispersed in the continuous polyolefin matrix phase impart flexibility and impact resistance to injection molded articles formed of the blend. TPO compositions are frequently used to fabricate injection-molded articles that are exposed to extreme changes in weather and temperature such as, for example, interior and exterior automotive parts (e.g., dashboard skins, airbag covers, bumper covers, exterior fascia, air dams and other trim pieces).
In some applications, TPO compositions can be used as a lower cost alternative to engineering resins such as acrylonitrile-butadiene-styrene (“ABS”) rubber. Unfortunately, unlike ABS rubber, conventional TPO compositions tend to undergo significant post molding shrinkage, typically in the range of 1.0-2.0%. Moreover, the shrinkage behavior of most conventional TPO compositions is not isotropic, meaning that the amount of shrinkage differs between the longitudinal and transverse flow directions. Although this shrinkage behavior does not adversely affect the properties of the resultant injection-molded article, it does make the fabrication of articles with tight dimensional tolerances more difficult. Molds must be specifically fabricated to account for the shrinkage behavior of the particular TPO composition and injection molding conditions being used (e.g. the degree of cooling allowed prior to mold ejection) in order to yield a finished article within desired dimensional tolerances. This shrinkage problem is particularly troublesome where the manufacturer has incurred the expense to have molds fabricated for use with a particular TPO composition and molding process and subsequently desires to substitute a different TPO composition or alter the process (e.g. increase the cooling rate). The change in composition or process conditions may result in a changed degree of shrinkage, thus yielding parts that are out of dimensional tolerance.
Attempts have been made to use additives such as talc and other inorganic particulate materials to control the shrinkage problems associated with TPO compositions. Unfortunately, incorporation of an amount of talc and/or other inorganic particulate material effective to control the shrinkage behavior of TPO compositions generally leads to other problems such as a reduction in flexibility, a decrease in toughness and a degradation of the surface finish of the injection-molded article.
Another method of adjusting the shrinkage behavior of TPO compositions is to increase the amount of the alpha-olefin copolymer elastomer rubber component in the blend. This disadvantageously increases the cost of the TPO composition because the alpha-olefin copolymer elastomer is generally the most expensive component in the blend. It also can adversely affect the melt flow behavior of the TPO composition and can make dispersing the alpha-olefin copolymer elastomer rubber in the continuous polyolefin matrix phase more difficult.